The overall objective of this proposal is to understand MLL-AF4, also known as t(4;l1), leukemia. This is the most common leukemia in infants and has a poor prognosis; it is also seen in older children and adults. We have recently developed "knock-in" technology to produce murine embryonic stem (ES) MLL-AF4 cells. We demonstrated that MLL-AF4 fusion protein is anti-apoptotic in ES cells and blocks hematopoietic differentiation in more mature cells. We predict that mice with leukemia derived from MLL-AF4 ES cells will have the same specific phenotype seen in human MLL-AF4 leukemia. We will evaluate the hypothesis that the poor prognosis of human MLL-AF4 leukemia results from resistance to apoptosis; in humans, apoptosis occurs in good prognosis leukemias following growth factor deprivation or chemotherapy but not in MLL-AF4 leukemia. We have three revised specific aims: (1) To evaluate the hypothesis that fusion gene is anti-apoptotic and the AF4 fusion gene is critical for this effect. Differentiation will be studied using embryoid body and hematopoietic colony culture systems. Conditional MLL-AF4 expression systems will be developed to control gene expression and permit survival of viable progenitor cells. (2) To evaluate the hypothesis that the effects of MLL-AF4 fusion gene are cell-type specific. (3) To study mice with MLL-AF4 or MLL-AF9 fusion gene leukemia.